A submerged plasma is a non-equilibrium plasma in contact with a liquid. A submerged plasma is formed by causing a discharge in a gas that is in contact with a liquid, thereby partially ionizing the gas. The gas for forming a plasma by a discharge may be introduced into a space in contact with the surface of a liquid (liquid surface) or may be introduced as bubbles into a liquid. Electrons, ions atoms and molecules in a submerged plasma may interact with a liquid or substances in the liquid through the gas-liquid interface, causing various chemical reactions.
Techniques have been under development for forming a plasma in a liquid in order to improve the water quality or to treat waste liquids. A practical object of such a submerged plasma technique is for example, the decomposition and sterilization of a low-degradable organic substance in an aqueous solution. A low-degradable organic substance, such as dioxin, that cannot be decomposed by ozone can be decomposed by hydroxyl radicals (.OH) formed by a reaction between a plasma and water. This is called an advanced oxidation process, and methods using ultraviolet rays, methods using hydrogen peroxide, etc., are also known in the art.
Discharge for producing a plasma in a liquid is generally classified into direct-current or low-frequency discharge and high-frequency or microwave discharge. Examples of the former include streamer and arc discharge in water using direct-current pulse discharge (see Non-Patent Document No. 1), and dielectric barrier discharge (see Patent Document Nos. 1, 2 and 3). With dielectric barrier discharge, a as is provided between electrodes, and a discharge plasma is induced in the gas. The devices disclosed in Patent Document Nos. 1, 2 and 3 all have a structure in which a 2-phase region of bubbles and water is sandwiched between electrodes. Examples of the latter include RB discharge and microwave discharge. Also in this case, bubbles are employed. Bubbles having been introduced or produced in a liquid, absorb electromagnetic energy, thereby producing a plasma (see Non-Patent Document Nos. 2 and 3). Submerged discharge techniques in general are reviewed in Non-Patent Document No. 2. The disclosures of Non-Patent Document Nos. 1 to 3 are hereby incorporated by reference in their entirety.